These studies propose to examine the development of B lymphocytes from their precursors in human bone marrow. Density gradient centrifugation and immunoadsorption techniques will be used to enrich marrow cells for the lymphoid subpopulation and deplete pre-formed B lymphocytes. I will culture the cells under a variety of conditions and monitor the appearance to cells bearing surface immunoglobulin and other markers of B lymphocytes. The proposed studies are designed to directly test the hypothesis that pre-B cells, defined using immunofluorescence, are the immediate precursors of B lymphocytes in human bone marrow.